1. Field of the Invention
This invention relates to sensors for determining oxygen saturation and hematocrit of the blood as it flows through an extracorporeal blood circuit in which it is oxygenated during bypass surgery and the like.
2. Description of the Prior Art
Fiber-optic sensors for the determination of oxygen saturation (amount of oxygenated hemoglobin) and/or the determination of hematocrit (amount of red blood cells), in the blood in an extracorporeal circuit or otherwise are disclosed in U.S. Pat. Nos. 4,444,498, 4,447,150, 4,651,741, 4,745,279, 4,776,340 and others. Many such sensors function by providing light to the blood sample via a fiber-optic cable, measuring the amount of reflected or transmitted light and calculating oxygen saturation or hematocrit based on those measurements. Light emitting diodes (LEDs) have been used in these devices to provide radiation at given wavelengths to the sample via the fiber-optic input cables; other fiber-optic cables have transmitted the light reflected by or transmitted through the sample at those wavelengths to photodiodes to produce a current proportionate to the reflected or transmitted light. In general, the LEDs alternately, or in series, emit light of one wavelength and then another. See Heinemann, U.S. Pat. No. 4,444,498, Lavallee, U.S. Pat. No. 3,647,299, Goldring, U.S. Pat. No. 4,684,245.
With respect to hematocrit, it has been calculated using a ratio of the amount of light reflected from the blood at two different distances from the light source; the isobestic point for oxygen saturation (about 810 to 820 nm) is the wavelength suggested because it is not influenced by the oxygen saturation level. Two detectors are used to receive the reflected light at two different distances from the source, all as disclosed in Moran, U.S. Pat. No. 4,776,340. In Karkar, U.S. Pat. No. 4,745,279, hematocrit in a blood circuit is measured by diffusion of light downstream from an LED source.
In sensors for oxygen saturation, often called oximeters, oxygen saturation has been determined by using the following ratio with light input at about 665 nm and about 820 nm. EQU Ratio=R.sub.820 /R.sub.665.
665 nm is a point of large difference between absorption of deoxyhemoglobin and oxyhemoglobin; 820 nm or thereabouts is the isobestic point at which the absorption is identical for both compounds.
A problem is that ambient light can interfere with the measurement to produce misleading results when there is a break in the cable or when the cable or the instrument is disconnected. Oxygen saturation and hematocrit readings will still be displayed, but are erroneously based on the amount of ambient radiation coming down the cable. In Karkar, U.S. Pat. No. 4,745,279, mentioned above, the hematocrit measurement is corrected by data received from a compensation sensor not exposed to the blood. The compensation sensor is connected in series with the hematocrit sensor and the ratio of their readings taken to produce a signal for the raw hematocrit measurement, to compensate for problems arising from temperature, optical noise, drive current, and aging of the light-emitting diodes.
In Nichols, et al., U.S. Pat. No. 4,603,700, a monitoring circuit is disclosed which reads the voltage at the detector (after exposure to the sample) and digitizes it. When the voltage is unexpected (for example, due to a disconnected probe), an appropriate error message is displayed such as "no probe" and the program is restarted. Heinemann, U.S. Pat. Nos. 4,444,498 and 4,447,150, discloses a system in which a switch creates a pull-up resistor to give grossly, and obviously erroneous, reading, until a cuvette which has been inadvertently detached is reattached.
Sperinde, et al., U.S. Pat. No. 4,523,279, discloses an oximeter catheter apparatus which measures the reflected intensity of the wavelength of interest and compares it to previously-stored and continually revised intensity levels, to minimize the effect of momentarily high radiation pulses due to changes of catheter position in the blood vessel.
Sperinde, et al., U.S. Pat. No. 4,453,218, discloses a signal filter method and apparatus in which a separate light source is added. The reflected reading from that source is used to calibrate and continually correct a baseline signal which is then used to create a maximum signal level. Where the incoming readings from the regular light source are higher than the maximum signal level, the system does not read them in order to avoid erroneously high readings.
This description of art is not intended to constitute an admission that any patent, publication or other information referred to is "prior art" with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R..sctn.1.56(a) exists.
A problem with some of the above devices is that accurate but unexpectedly high signals may, in fact, be discarded by the correction mechanism. In other devices, high signals may be thought to be accurate rather than indicative of a problem. Some of the devices require the addition of an additional light source and others require the inclusion of complex monitoring and calculating functions in the device, or at least in the software, in order to function. Some are designed particularly for use with catheters and are not appropriate for use with oximeter devices adapted to extracorporeal blood circuits. it would, thus, be desirable to provide a device which eliminates some of these difficulties.